No, a lay witness would not ordinarily be able to make an expert opinion or judgment. His qualifications as an expert and whether to accord him the right to make an expert opinion could have been challenged by the defendant's attorney in a qualifications examination called a voir dire.
Perhaps this didn't happen, and the lay testimony was accepted as expert opinion.
But there is a "gray area" where perhaps a lay witness was just making an observation about what he saw or heard that really WASN'T a medical diagnosis or opinion. Something like "I saw the victim lying on the ground and he appeared to be unconscious, did not respond to our calling his name, had no pulse, wasn't breathing, he had bluish skin and blood was trickling out of his mouth", is not expert testimony, as the witness is not concluding whether he was asphyxiated, had a concussion or was even alive or dead".
If the witness was asked "do you think he was alive when you saw him?" that also might not require a physician or medical professional to offer a medical opinion or diagnosis, it is just a valid eyewitness testimony of what an ordinary person with reasonable perception skills who has seen live and dead people might conclude. Ditto with whether someone was under the influence of some drug or alcohol, or had tremors or seizures, etc.
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